The 25-Hour Day Is Coming

New measurements suggest that the length of a day on Earth is increasing — albeit very slowly. In 200 million years, whatever life forms inhabit this planet will experience something humans often fantasize about — an extra hour. Every. Single. Damn. Day.

Researchers at Durham University and the U.K.’s Nautical Almanac Office looked at records of eclipse sightings as far back as 720 B.C. to analyze how quickly Earth is losing rotational energy as it spins around its axis. Their results, published Wednesday in the Proceedings of the Royal Society, found that the length of a day increases by 1.8 milliseconds every century.

That is a tiny, tiny change, but it adds up over millennia. The U.K. astronomers used a computer model to estimate where on Earth past eclipses would be visible, assuming perfectly consistent day lengths. Then they compared those results with actual records of eclipse sightings, including reports from ancient Babylon, China, and Greece.

In 720 B.C. the length of a single Earth day would have been just 50 milliseconds shorter than today. That might seem insignificant, but add up those milliseconds over 2,700 years and you get almost 14 hours added compared to what you’d expect for an unchanged day length. That’s enough to spin Earth more than halfway around on itself, and completely reverse expectations for who on the planet will have front row seats to an eclipse event.

Previous work suggested that Earth days had been slowing at an even faster rate — 2.3 milliseconds per century. But that work modeled only tidal friction, which is the loss of rotational energy due to Earth’s massive oceans sloshing around in an interaction with the moon’s gravity.

This is a significant source of regulation for this planet’s rate of spin, but it isn’t the only one. Many factors influence the length of an Earth day, including climate. When the planet is in a deep freeze, more mass is held close to the poles, which increases the speed of rotation. In warmer times, the planet bulges out more around the equator, which — like a figure skater holding out his arms — slows the spin. This observational eclipse data is important because it necessarily takes into account all of the forces interacting to change Earth’s rotational rate.

We tend to think of a day as a basically unchanging unit of time, and relevant to our lives, it is. But it hasn’t always been that way. One new study suggests that the collision between Earth and a Mars-sized megarock 4.5 billion years ago left this planet spinning at an incredible rate of once every two hours. That event nearly obliterated both space rocks and birthed the Earth-moon system we know today. It’s taken a gradual slowing over 4.5 billion years to get the 24-hour day we know and love so well. Adjusting to a 25-hour doesn’t sound nearly as stressful.

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